1. Field of the Invention
The present invention relates to a stepping motor structure, and more specifically to a low-profile stepping motor having a reduced dimension in the axial direction.
2. Description of the Related Art
Recently, higher performance is increasingly required in various electronic devices such as a printer, a facsimile, and a floppy™ disk drive, and accordingly there is a growing requirement for a stepping motor provided both with a high performance such as a high torque and with a high quality such as a low cogging torque and a low vibration. PM (permanent magnet) type stepping motors, which satisfy the requirement to some extent, have been extensively used for the electronic devices.
FIGS. 1 to 5 explain a conventional PM type stepping motor extensively used, wherein FIG. 1 is a perspective view thereof partly cutaway, FIG. 2 is a cross-sectional (partly) view of one stator unit thereof, FIG. 3 is an exploded perspective view of a stator assembly thereof, FIG. 4 is an axial cross-sectional view thereof, and FIG. 5 is a developed view of an inner circumference (pole teeth) of a stator assembly thereof.
In FIG. 1, illustrated are: a front plate 1 punched out of a stainless steel plate; a front bearing 2 made of oil-impregnated alloy; stator yokes 3, 3′ each punched out of a soft magnetic steel plate and formed into a doughnut shape with pole teeth 10 bent up at its inner circumference; coils 4, 4 each comprising a bobbin 5 with a polyurethane-coated copper wire wound therearound; two stator units 6, 6′ each housing the coil 4 and coaxially attached to each other thus constituting a stator assembly 7; a rotary shaft 8 as a center of rotation; a field rotor magnet 9 facing the pole teeth 10 of the stator yokes 3, 3′ and having a plurality of magnetic poles 11 (see FIG. 4) on its outer circumference; and a sleeve 12 jointing the rotor magnet 9 and the shaft 8 thus constituting a rotor assembly 22 (see FIG. 4).
Referring to FIG. 3, the stator units 6, 6′ each comprise the stator yokes 3 and 3′ whose respective pole teeth 10 are arranged so as to intermesh with each other with a constant gap therebetween as shown in FIG. 2 when the stator yokes 3 and 3′ are coupled to each other, wherein the pole teeth 10 of the stator yoke 3 are shifted from the pole teeth 10 of the stator yoke 3′ by an electrical angle of 180 degrees. The two stator units 6 and 6′ constituting the stator assembly 7 are joined to each other coaxially by a molding method with a polymeric material, or by a plasma welding method such that respective intermeshing pole teeth 10 of the stator unit 6 and 6′ are shifted from each other by an electrical angle of 90 degrees.
Referring to FIG. 4, the rotor assembly 22 is structured such that the shaft 8 as rotation center axis is press-fitted in the sleeve 12, which is inserted in the field rotor magnet 9 and adhesively fixed thereto so as to be concentric therewith. The rotor assembly 22 thus structured with the field rotor magnet 9 multi-pole magnetized is rotatably held inside the stator assembly 7 with the rotary shaft 8 inserted through the front bearing 2 and a rear bearing 16 both made of oil-impregnated alloy and fixed respectively to the front plate 1 and a rear plate 15, such that the field rotor magnet 9 with the plurality of magnetic poles 11 faces the inner circumference (defined by the pole teeth 10) of the stator assembly 7 with a small air gap in-between so as to be concentric with each other. The front and rear plates 1 and 15 are fixedly attached respectively to the stator 6 and 6′ by, for example, plasma welding.
In FIG. 5, one example of the pole teeth 10 of the stator units 6 and 6′ is shown as developed. In this example, the stator units 6, 6′ each have ten pole teeth 10 in a space angle of 360 degrees, and the pole teeth 10 have a pitch interval identical with the width (not shown in the figure) of the magnetic poles 11 of the rotor magnet 9.
The aforementioned electronic devices are increasingly downsized and lower-profiled, and therefore the components incorporated in the devices, including a PM type stepping motor, are required to be downsized and lower-profiled. Especially, a PM type stepping motor is strongly requested to have its profile lowered even if it results in an increased horizontal area dimension. However, as known from FIGS. 1 and 4, the conventional PM stepping motor described above has two stator units 6, 6′ stacked on each other in an axial direction, which puts limitations on decreasing the axial dimension, namely the profile.